Fixed common depth - All sensors are grouped effectively at the same depth which is effectively fixed for the duration of the series

Sensor Depth Datum

Sea floor reference - Depth measured as a height above sea floor but converted into a depth relative to the sea surface according to the same datum as used for sea floor depth (applicable to instrument depths not bathymetric depths)

Sea Floor Depth Datum

Instantaneous - Depth measured below water line or instantaneous water body surface

Parameters

BODC CODE

Rank

Units

Short Title

Title

AADYAA01

1

Days

Date(Loch_Day)

Date (time from 00:00 01/01/1760 to 00:00 UT on day)

AAFDZZ01

1

Days

Time(Day_Fract)

Time (time between 00:00 UT and timestamp)

LCDAEL01

1

Degrees True

CurrDir

Current direction (Eulerian) in the water body by in-situ current meter and correction to true North

LCSAEL01

1

Centimetres per second

CurrSpd_CM

Current speed (Eulerian) in the water body by in-situ current meter

Definition of Rank

Rank 1 is a one-dimensional parameter

Rank 2 is a two-dimensional parameter

Rank 0 is a one-dimensional parameter describing the second dimension of a two-dimensional parameter (e.g. bin depths for moored ADCP data)

Open Data supplied by Natural Environment Research Council (NERC)

Aanderaa Recording Current Meter Model 4/5

Manufacturer's specifications: Meter (recording unit: height 51cm, diameter 12.8cm, vane size 37x100cm; overall: length 137cm, height 75cm) is designed for depths down to 2000m (6000m RCM model 5). It incorporates a spindle which is shackled into the mooring line. The meter is attached to the spindle through a gimbal mounting which permits a maximum 27° deviation of the spindle from the vertical, the meter still remaining horizontal.

Meter comprises :-

Savonius rotor magnetically coupled to an electronic counter - the number of revolutions during the sampling interval giving the average current speed over the interval - starting speed 2cm/s (users find 1.5 to 3cm/s), range 2.5 to 250cm/s, accuracy greater of 1cm/s or 2 per cent.

Vane, which aligns instrument with current flow, has a balance weight ensuring static balance and tail fins to ensure dynamic balance in flows up to 250cm/s.

Magnetic compass (needle is clamped to potentiometer ring at instant of sampling only) - direction recorded with 0.35° resolution, 5° accuracy (1.5° claimed by MAFF, Lowestoft) for speeds 5 to 100cm/s, 7.5° accuracy for remaining speeds within 2.5 to 200cm/s range, maximum compass tilt (i.e. maximum deviation of the meter from the horizontal at which the meter still registers correctly) is 12° in both pitch and roll axes.

meters with digital measurement of speed i.e. utilizing an electronic reed switch to count the total number of rotor revolutions during the sampling interval:

speed = 1.5 + 42 * B * (M/T) cm/s (2)

all meters: direction = 1.5 + 0.349N ° magnetic (3)

where B is the number of rotor revolutions per count, M (bits) binary is the count over the sampling interval T (sec) and N (bits) binary is the direction reading.

Note: Data collecting laboratories may calibrate their own meters and so not use the manufacturer's calibration equations.

BODC Current Meter Screening

BODC screen both the series header qualifying information and the parameter values in the data cycles themselves.

Header information is inspected for:

Irregularities such as unfeasible values

Inconsistencies between related information. For example:

Depths of meter and sea bed.

Times for mooring deployment and for start/end of data series.

Length of record or number of data cycles, the cycle interval, the clock error and the period over which accrued.

Parameters stated as measured and the parameters actually present in the data cycles.

Originator's comments on meter/mooring performance and data quality.

Documents are written by BODC highlighting irregularities which cannot be resolved.

Data cycles are inspected using time series plots of all parameters. Currents are additionally inspected using vector scatter plots and time series plots of North and East velocity components. These presentations undergo intrinsic and extrinsic screening to detect infeasible values within the data cycles themselves and inconsistencies as seen when comparing characteristics of adjacent data sets displaced with respect to depth, position or time. Values suspected of being of non- oceanographic origin may be tagged with the BODC flag denoting suspect value.

The following types of irregularity, each relying on visual detection in the time series plot, are amongst those which may be flagged as suspect:

If a large percentage of the data is affected by irregularities, deemed abnormal, then instead of flagging the individual suspect values, a caution may be documented. Likewise documents will highlight irregularities seen in the current vector scatter plots such as incongruous centre holes, evidence of mooring 'knock-down', abnormal asymmetry in tidally dominated records or gaps as when a range of speeds or directions go unregistered due to meter malfunction.

The term 'knock-down' refers to the situation when the 'drag' exerted on a mooring at high current speeds may cause instruments to tilt beyond the angle at which they are intended to operate. At this point the efficiency of the current sensors to accurately record the flow is reduced.

Inconsistencies between the characteristics of the data set and those of its neighbours are sought, and where necessary, documented. This covers inconsistencies in the following:

Maximum and minimum values of parameters (spikes excluded).

The orientation and symmetry of the current vector scatter plot.

The direction of rotation of the current vectors.

The approximate amplitude and periodicity of the tidal currents.

The occurrence of meteorological events and, finally, for series for which no time check was possible, the phase.

This intrinsic and extrinsic screening of the parameter values seeks to confirm the qualifying information and the source laboratory's comments on the series. In screening and collating information, every care is taken to ensure that errors of BODC making are not introduced.

Data Processing Notes

The data were low-pass filtered, the transmittance of the filter is close to 1 for 0 to 12 cycles per day and close to 0 at 24 cycles per day. The data were then thinned by 3 to give nominally half-hourly values.

Mooring and Data Processing Information

Mooring Details

The meters were deployed on an I-shaped mooring, with an anchor on the sea floor leading by means of a wire rope to the release gear and command pinger, leading to the in-line current meters, a pinger and finally, the subsurface buoy. On occasion a sea floor pressure recorder was incorporated in the mooring system.

Data Processing

The data were translated from tape to cards and obviously incorrect values (i.e. zeros) were replaced by interpolation. Dead spaces between 1024 and 0 on the speed series were linearly interpolated. The data were then copied to disk, calibrated to m/sec and true direction and resolved to u(north component) and v(east component).